There are already known various constructions of optical waveguides, such as optical fibers. Such optical waveguides guide light in a predetermined path, such as longitudinally through the optical fiber core. Depending on the cross-sectional area or diameter of the core with respect to the wavelength of the light launched into the core, the light is guided in the core in a single mode or in a multimode manner.
In some applications, it would be beneficial to be able to discriminate between the modes of light of a given frequency propagating in the waveguide core, that is, to treat them differently by reflecting at least one of such modes back to and out of the launching end of the core while letting the remaining mode or modes continue to propagate to the other end of the core in a substantially undisturbed manner. Yet, no one has proposed so far that and how this mode discrimination could be accomplished during the passage of the light of the given frequency in the core of a multimode optical waveguide or fiber.
It is also already known, for instance from the commonly owned U.S. Pat. No. 4,725,110, issued on Feb. 16, 1988, to impress periodic grating elements into the optical fiber core by exposing the core through the cladding to the interference pattern of two compatible coherent ultraviolet light beams that are directed against the optical fiber at two angles relative to the fiber axis that complement each other to 180.degree.. This results in a situation where the grating elements, which are constituted by periodic index of refraction variations permanently induced in the core by exposure to the ultraviolet radiation interference pattern, are oriented normal to the fiber axis and are equidistantly longitudinally spaced from one another so that the grating consisting of such grating elements reflects, of the light launched into the fiber core for guided single mode propagation therein in a propagation direction, only that having a wavelength within a very narrow range, back along the fiber axis opposite to the original propagation direction so that such reflected light is guided in the core to the point at which the original light had been launched into the fiber core. On the other hand, this grating is transparent to single mode light at wavelengths outside the aforementioned narrow band so that it does not affect the further propagation of such other light. This patent, however, does not mention the possibility of using this technique for providing permanent grating elements of the above kind in the core of a multimode optical fiber and, consequently, does not address the consequences of the use of this technique in conjunction with multimode optical waveguides and the effect of the grating region produced in this manner on the propagating light of a given frequency.
Accordingly, it is a general object of the present invention to avoid the disadvantages of the prior art.
More particularly, it is an object of the present invention to provide an optical filter which does not possess the disadvantages of the known filters of this kind.
Still another object of the present invention is so to develop the optical filter of the type here under consideration as to be capable of discriminating between diverse transverse spatial modes of light at a given frequency.
It is yet another object of the present invention to devise an optical filter of the above type which reflects one mode of the light of the given frequency back to the point of origin while permitting substantially undisturbed passage of the remaining mode or modes of such light therethrough.
A concomitant object of the present invention is to design the mode discrimination optical filter of the above type in such a manner as to be relatively simple in construction, inexpensive to manufacture, easy to use, and yet reliable in operation.
Yet another object of the present invention is to accomplish the mode discrimination during the passage of the light through an optical waveguide.